This invention relates to methods of cross linking polysaccharides to form cross linked polysaccharides, and more particularly, to viscosified treatment fluids that are self-breaking. More particularly, the present invention provides “dual functional components” that are chemical compositions that have a dual functionality when interacting with polysaccharides in an aqueous treatment fluid.
Polysaccharides are carbohydrates in which tens, hundreds, or even thousands of simple sugars are linked together. Since they have no free anomeric hydroxyls (except for one at the end of the chain), they are not reducing sugars and do not show mutarotation. Cellulose, starch, and various galactomannans are some of the most-widely occurring polysaccharides. One common gelling agent used in subterranean treatment fluids is guar, a galactomannan type of polysaccharide which may be cross linked to yield a high gel strength for suspension, and yet relatively easily broken when desired (that is, the viscosity of the treatment fluid viscosified with guar may be reduced when desired). Because of its abundance, price, and geometry favorable to cross linking, guar is the most commonly used gelling agent in treatment fluids. Polysaccharides are commonly used to viscosify aqueous fluids to create, e.g., viscosified treatment fluids that inhibit particle settling by virtue of viscosity. If the polysaccharide is cross linked, these viscosified treatment fluids can approach near zero particle settling rates.
Viscosified treatment fluids are often used in industries such as the mining, explosive, and petroleum industries. Treatment fluids may be used in a variety of subterranean treatments, including, but not limited to, stimulation treatments and sand control treatments. As used herein, the term “treatment,” or “treating,” refers to any operation that uses a fluid in conjunction with a desired function and/or for a desired purpose. The term “treatment,” or “treating,” does not imply any particular action by the fluid or any particular component thereof.
In subterranean operations, one common production stimulation operation that employs a treatment fluid is hydraulic fracturing. Hydraulic fracturing operations generally involve pumping a treatment fluid (e.g., a fracturing fluid) into a well bore that penetrates a subterranean formation at a sufficient hydraulic pressure to create or enhance one or more cracks, or “fractures,” in the subterranean formation. The fracturing fluid may comprise particulates, often referred to as “proppant,” that are deposited in the fractures. The proppant particulates, inter alia, prevent the fractures from fully closing upon the release of hydraulic pressure, forming conductive channels through which fluids may flow to the well bore. Once at least one fracture is created and the proppant particulates are substantially in place, the fracturing fluid may be “broken” (i.e., the viscosity is reduced), and the fracturing fluid may be recovered from the formation.
Treatment fluids are also utilized in sand control treatments, such as gravel packing. In gravel-packing treatments, a treatment fluid suspends particulates (commonly referred to as “gravel particulates”) for delivery to a desired area in a well bore, e.g., near unconsolidated or weakly-consolidated formation zones, to form a gravel pack to enhance sand control. One common type of gravel-packing operation involves placing a sand control screen in the well bore and packing the annulus between the screen and the well bore with the gravel particulates of a specific size to prevent the passage of formation sand. The gravel particulates act, inter alia, to prevent the formation particulates from occluding the screen or migrating with the produced hydrocarbons, and the screen acts, inter alia, to prevent the particulates from entering the production tubing. Once the gravel pack is substantially in place, the viscosity of the treatment fluid may be reduced to allow it to be recovered. In some situations, fracturing and gravel-packing treatments are combined into a single treatment (commonly referred to as “frac pack” operations). In such “frac pack” operations, the treatments are generally completed with a gravel pack screen assembly in place with the hydraulic fracturing treatment being pumped through the annular space between the casing and screen. In this situation, the hydraulic fracturing treatment may end in a tip screen-out condition. In other cases, the fracturing treatment may be performed prior to installing the screen and placing a gravel pack.
Maintaining sufficient viscosity in these treatment fluids is important for a number of reasons. Maintaining sufficient viscosity is important in fracturing and sand control treatments for particulate transport and/or to create or enhance fracture width. Also, maintaining sufficient viscosity may be important to control and/or reduce fluid-loss into the formation. Moreover, a treatment fluid of a sufficient viscosity may be used to divert the flow of fluids present within a subterranean formation (e.g., formation fluids, other treatment fluids) to other portions of the formation, for example, by “plugging” an open space within the formation. At the same time, while maintaining sufficient viscosity of the treatment fluid often is desirable, it also may be desirable to maintain the viscosity of the treatment fluid in such a way that the viscosity may be reduced at a particular time, inter alia, for subsequent recovery of the fluid from the formation. Additionally, the viscosity also may help determine the open fracture width.
To increase the viscosity of the viscosified fluid, the polysaccharide component of the fluid may be cross linked. The term “cross linked” as used herein refers to bonds between two or more molecules of given polysaccharide(s). Historically, there have been few good ways to make a sufficient cross linked microbial polysaccharide gel.
Also, the cross linking behavior of such conventional cross linking agents may become inhibited by components in the treatment fluid. For example, the composition of the water component of an aqueous treatment fluid can interfere with the cross linking behavior of conventional cross linking agents. Moreover, such conventionally cross linked polysaccharides may be heavily dependent on conditions such as pH and temperature. Because of this dependency and resultant instability, the viscosified treatment fluid may lose its viscosity prematurely, for instance, the proppant or gravel can drop out of a conventional viscosified treatment fluid before it has been placed in the desired interval in the subterranean formation neighboring the well bore.
At some point in time, e.g., after a viscosified treatment fluid has performed its desired function, the viscosity of the viscosified treatment fluid should be reduced. This is often referred to as “breaking the gel” or “breaking the fluid.” This can occur by, inter alia, reversing the crosslink between cross linked polymer molecules, breaking down the molecules of the polymeric gelling agent, or breaking the crosslinks between polymer molecules. The use of the term “break” herein incorporates at least all of these mechanisms. Certain breakers that are capable of breaking treatment fluids comprising cross linked gelling agents are known in the art. For example, breakers comprising sodium bromate, sodium chlorite, and other oxidizing agents have been used to reduce the viscosity of treatment fluids comprising cross linked polymers. Examples of such breakers are described in U.S. Pat. No. 5,759,964 to Shuchart, et al., and U.S. Pat. No. 5,413,178 to Walker, et al., the relevant disclosures of which are herein incorporated by reference. However, many of these breakers are only effective in reducing the viscosity of a treatment fluid at neutral-to-alkaline pH levels (e.g., above about pH 6). Excessive concentrations of those breakers and/or additional catalysts may be required to effectively reduce the viscosity of a treatment fluid at lower pH levels (e.g., below about pH 6). One should note that low pH may break some gels in some circumstances. High concentrations of breaker and/or additional catalysts may be problematic since they may, among other things, increase the cost and complexity of a treatment fluid, adversely affect other components of the treatment fluid, and/or leave damaging residues in the subterranean formations where they are used.
Thus, there are needs for improved subterranean formation treating fluids and methods whereby the fluids are not thermally unstable, do not produce insoluble residues, have high proppant carrying capacities, produce easily removed filter cake (or no filter cake), do not have to be hydrated in holding tanks for long periods of time, can have their properties changed during use, and can be recovered and reused if desired. Moreover, to avoid the problems associated with conventional breakers, it would be desirable to have cross linked polysaccharides that, inter alia, are “self-breaking,” meaning that they can break at a desired time without the need for additional breakers.